Due to the high thermal efficiency achieved by compression-ignited engines (for example, in comparison with spark-ignited engines), these engines are commonly utilized in industrial applications. The high efficiency of compression-ignited engines, such as diesel engines, is due in part to the ability to use higher compression ratios than spark-ignited engines (such as gasoline engines) as well as the ability to control power output without a throttle. In the latter regard, the lack of a throttle eliminates throttling losses of premixed charges typical in spark-ignited engines thereby resulting in significantly higher efficiency at partial load. However, compression-ignited engines and diesel engines in particular typically cannot achieve the low oxides of nitrogen (NOx) and particulate emission levels that are possible with spark-ignited engines. Diesel engines typically inject diesel fuel into the engine's combustion chamber when that chamber's piston is near the end of the compression stroke. The high pressure present in the chamber ignites the diesel fuel. Due to this mixing controlled nature of diesel combustion, a large fraction of the fuel exists at a very fuel-rich equivalence ratio. That is, the fuel and air in the combustion chamber are not necessarily a homogenous mixture. This can result in incomplete combustion of the diesel fuel, which tends to result in high particulate emissions. Furthermore, the fuel-rich equivalence ratio can also lead to high flame temperatures in the combustion process, which results in increased NOx emissions.
As tougher environmental standards are being enacted for diesel sources, users of diesel engines are looking for ways to lower emissions. One solution is to reduce the amount of diesel injected into the combustion chamber, which reduces the equivalence ratio and works to reduce particulate and NOx emissions; however, it also reduces engine power. In order to reduce particulate and NOx emissions levels from diesel engines, such engines can also be partially or completely converted for use with gaseous fuels such as, compressed natural gas (CNG), liquid natural fuels (LNG) such as ethanol, and liquid or liquefied petroleum gas (LPG), such as propane. Utilization of such gaseous fuels with diesel engines not only provides for more complete combustion and thereby enhanced fuel economy, but also typically results in lower engine emissions. However, gaseous fuels typically do not have the cetane value required to allow for their ignition through compression. Accordingly, diesel engines must be modified to use such fuels.
Methods for converting a diesel engine to consume gaseous fuels typically fall into three categories. The first is to convert the engine to a spark-ignited engine. A second is to convert the engine to allow for the direct injection of gaseous fuels into the combustion chamber with injected diesel. A third is a dual-fuel technology, in which the gaseous fuel is mixed with all or a portion of the intake air of the engine. The second and third methods utilize injected diesel (namely, pilot diesel) to ignite the gaseous fuel. In this regard, the combustion of the gaseous fuel results in more complete combustion of the diesel. Furthermore, as the gaseous fuel allows the engine to produce additional power, less diesel fuel is injected into the cylinders
Conversion to a spark-ignition system and/or a direct gaseous fuel injection system for utilizing gaseous fuels with a diesel engine each typically require substantial modification to the diesel engine. Such modifications can include replacement of cylinder heads, pistons, fuel injection system and/or duplication of many engine components (for example, injection systems). Accordingly, these systems are typically expensive and oftentimes unreliable. On the other hand, dual-fuel systems require little modification to existing engines. Dual-fuel operation, in which gaseous fuels are mixed with intake air prior to the introduction of that air into the engine, is known in the art as injection. According to one method, gaseous fuel is injected in an intake channel and/or an intake port for each cylinder. This is commonly termed “gaseous fuel port injection” and can comprise 1-2 injectors per cylinder. According to a further method, gaseous fuel is injected in a mixer unit, which can be located before or after the throttle, or at the entrance to an intake pipe. The latter case is common if a throttle is not used. This method is commonly termed “mixer injection” or “central mixer” and can comprise 4-12 injectors in the mixer unit.
The mixture of gaseous fuel and intake air is introduced into each cylinder of the engine during the intake stroke. During the compression stroke of the piston, the pressure and temperature of the mixture are increased. Near the end of the compression stroke, a small quantity of pilot diesel fuel from the engine's existing diesel fuel injection system is injected into the cylinder. The pilot diesel ignites due to compression and in turn ignites the mixture of gaseous fuel and intake air. Such injection systems can be retrofitted onto existing diesel engines with little or no modification of the existing engine. Furthermore, engines using such injection systems can typically be operated in a dual-fuel mode or in a strictly diesel mode (for example, when gaseous fuel is not available).
Dual-fuel systems have suffered a number of disadvantages that have prevented widespread use of such systems. The first disadvantage is typically encountered at high load operating conditions when elevated temperature and pressure in the engine during the compression strokes makes the intake air/gaseous fuel mixture susceptible to premature detonation or knocking. Furthermore, at such high loads, some gaseous fuels (for example, natural gas) lack the thermal energy (namely, BTUs) required to maintain a desired power output of the engine. Another limitation can be availability of air from a turbocharger due to increased boosting pressure from the added gaseous fuel volume. Overcoming these disadvantages may involve reducing gaseous fuel content in order to reach desired engine power, redesigning the boost system or limiting engine performance. Another disadvantage is encountered at low engine load, where the gaseous fuel and air mixture may be too lean for satisfactory combustion. In this instance, fuel consumption can actually increase, as can the emissions of hydrocarbons (namely, unburned gaseous fuels) and particulates.
A further disadvantage is related to impurities or pollutants in the gaseous fuel. A dual fuel engine is usually configured to operate using a fuel having a certain quality. If a substandard or lower grade gaseous fuel is injected, this will invariably cause knocking. Each of these problems can be broadly termed a gaseous fuel metering problem of a gaseous fuel flow volume to the engine.
The above noted disadvantages are particularly acute in diesel engines, which run at varying load levels during operation. Such engines require the volume of gaseous fuel injected into the intake airflow to vary with the varying requirements or demands of the engine in order to maintain desired power and emission outputs.
U.S. Patent Application Publication No. 2007/125321 discloses a dual fuel engine mainly intended for stationary use, in which a knock sensor is used for controlling the engine and the supply of gaseous fuel. In order to avoid a loss of power if knock is detected, the gaseous fuel is immediately reduced to zero or near zero and the amount of diesel is increased correspondingly. The amount of gas is then increased gradually to a lower level than the original. This method of operation will have noticeable effects on engine noise and behavior. This type of engine control is impractical for vehicular applications, where the engine will be subjected to numerous transients and changes in power requirements.
The present dual fuel engine system has control functionality that can be installed on engines in vehicles, for transient engine operation. The present dual fuel engine system reduces emissions of NOx and particulates from the engine. In the present system, gaseous fuel is provided to a diesel engine based on the varying requirements or demands of the engine, and operation of the engine on different grades of gaseous fuels is enabled.